The need to remove heat from gas streams arises in numerous applications. In motor vehicles, for instance, it may be necessary to remove heat from the intake and/or exhaust gas streams. For example, intake air (or “charge air”) requires cooling in some applications, for example in turbocharged or supercharged engines. In vehicles incorporating exhaust gas recirculation (EGR) or exhaust gas heat recovery (EGHR) systems, heat is removed from the exhaust gas stream. The heat removed from the intake or exhaust gas stream is typically transferred to a liquid coolant in a heat exchanger.
In EGHR systems, for example, heat from vehicle exhaust gases is transferred to other vehicle components via a liquid coolant or oil in order to provide faster heating of air and vehicle fluids on start-up of the vehicle, thereby reducing fuel consumption. Heat extracted from the exhaust and used to heat up vehicle fluids such as engine oil and transmission fluid makes them less viscous and improves fuel economy during start-up. Also, heat extracted from the exhaust gases can be used for rapid heating of the passenger compartment and for window defrosting, reducing the need for long idling periods during start-up in cold weather. After the initial start-up period the recovery of heat from the exhaust gases is no longer required. Therefore, EGHR systems typically include a bypass to minimize heat transfer from the exhaust gases to the liquid coolant once the vehicle reaches normal operating temperature. This helps to minimize the load on the cooling system and minimizes the risk of boiling or thermal degradation of the liquid coolant.
An EGHR system therefore incorporates a gas to liquid heat exchanger for extracting heat from the vehicle exhaust gas and transferring the heat to a liquid coolant, typically a water/glycol engine coolant, although direct heat transfer to an oil is also possible. The EGHR system also includes a diverter valve for directing at least a portion of the exhaust gas flow through the heat exchanger during vehicle start-up, and for bypassing the heat exchanger once the heat from the exhaust gas is no longer required. The heat exchanger and the valve need to be connected to the exhaust gas system piping. An actuator is also provided in order to control operation of the valve. The valve may be operated by means of an electronically controlled solenoid, a wax motor, engine vacuum or a bimetal or shape memory alloy (SMA) actuator.
To save space and to reduce cost and vehicle weight, the valve and heat exchanger may be integrated into a single unit, referred to herein as an EGHR device. In many integrated EGHR devices, however, the heat exchanger is heated by the exhaust gases whether the device is in heat exchange mode or bypass mode. This may be due to exhaust gas leakage past the valve and/or thermal conduction. This increases the amount of heat transferred to the coolant, increasing the load on the cooling system, and risking cumulative thermal degradation of the coolant or induced thermal stresses which can cause damage to the heat exchanger.
There remains a need for simple and effective heat recovery devices for motor vehicle intake and exhaust gas systems which minimize usage of space, weight, and number of components, which are readily integratable into existing exhaust system piping, and which also minimize thermal stresses and unwanted heat transfer to the coolant in bypass mode.